Managing directors at Goldman Sachs openly insulted clients on internal emails, according to the comments of a high profile employee leaving the company.

Goldman Sachs' European equity derivatives boss, Gregory Smith, said he had seen emails from several managing directors that openly branded clients as "muppets".

Goldman Sachs denies the claims, and has said it supports customer success.

In a letter to the New York Times, Smith said he had chosen to leave the bank because its practices were "toxic and destructive".

"It makes me ill how callously people talk about ripping their clients off. I have seen five different managing directors refer to their own clients as 'muppets', sometimes over internal email," wrote Smith, who was an employee of the bank for over 11 years.

"I can honestly say that the environment now is as toxic and destructive as I have ever seen it."

Smith described the company as originally having a culture of teamwork and humility. He said this had virtually disappeared during the current tenure of chief executive Lloyd Blankfein, who once said he was "doing the work of God" - a comment Blankfein described as a joke that backfired. Smith wrote that the "decline in the firm's moral fibre" represented "the single most serious threat to its long-run survival".

Executive comments sent by email are always liable to be retrieved later, particularly in situations where disputes and lawsuits occur, though no emails have yet been publicly shown to back up Smith's claims. 

The news is not the first time Goldman Sachs executives have been described making damaging comments by email.

Michael Swenson, a former executive in Goldman Sachs' fixed income trading division, wrote a series of infamous emails in 2007, before the worst impact of the financial crisis. He wrote that the investment bank's traders were urged to "kill" rival investor positions, and cause "maximum pain", adding that "this will have people totally demoralised". Goldman Sachs insisted last year that the language "does not reflect the reality" of its trading at the time.

Today, the bank said in a statement in response to Smith's letter: "We disagree with the views expressed, which we don't think reflect the way we run our business."

It added: "In our view, we will only be successful if our clients are successful. This fundamental truth lies at the heart of how we conduct ourselves."